Preamplifiers Can Help To Produce Good-Quality Measurements

A preamplifier is often a low-noise amplifier (LNA) with moderate gain. If a preamplifier with low noise is used, the noise of all of the subsequent stages is reduced by its gain. In addition, the preamplifier’s noise is injected directly into the received signal. A preamplifier is needed to boost the desired signal power while adding minimal noise and distortion. The signal can then be retrieved in the system’s later stages. Such amplifiers are the focus of a 12-page white paper from Agilent Technologies (Santa Clara, CA) titled, “Agilent Preamplifiers and System Noise Figure.”

The paper begins by providing a description of noise and the effect that an amplifier has on the sensitivity and noise figure of a system. In microwave systems, noise can be generated from external sources or the system itself. A system’s noise level sets the lower limit on the signal’s magnitude, which can be detected in the presence of noise. To achieve optimal performance, a minimum residual noise level is needed.

The noise figure is used to determine the amount of noise that a specific component will add to the system. The paper provides a table to show how amplifiers can be added to noise-figure measurement systems to decrease the system noise figure. By placing an appropriate preamplifier in front of a spectrum analyzer, an engineer can obtain a system noise figure that is lower than that of the spectrum analyzer alone. System sensitivity also will improve.

Keep in mind that a preamplifier amplifies noise, however. This output noise can be higher than the effective input noise of the analyzer. Two extreme cases are given to show how preamplifiers improve sensitivity. In addition, graphics show how to determine the system noise figure using the noise figures of the spectrum analyzer and preamplifiers along with the amplifier gain.

In addition, the note offers guidance on choosing the right preamplifier. Essentially, this choice depends on whether the best sensitivity or widest measurement range are needed. Two cases are detailed. The application note closes by offering numerical examples using one of the company’s own preamplifiers. It also offers tips for using a preamplifier for noise measurements. The paper concludes that a low-noise preamplifier may be helpful if the device under test (DUT) has low or negative gain or the instrument has a high noise figure. A preamplifier should not be used, however, if a DUT already has significant positive gain.